The present invention relates to an apparatus and method for measuring the biopotential signals produced in a subject, and more specifically to an apparatus and method that is configurable to provide either a 1-channel operating mode or a mode resembling 2-channel operation.
Electroencephalography (EEG) is a well established method for assessing the brain function by picking up the weak biosignals generated in the brain with electrodes on the skull surface. To obtain the biosignals, multiple electrodes are placed on the scalp of a patient in accordance with a recognized protocol. EEG has been in wide use for decades in basic research of the neural system of brain as well as clinically in diagnosis of various neurophysiological disorders.
The EEG signals received by the electrodes from the scalp are amplified by amplifiers which may be of the differential type to minimize electrical interference. Each amplifier has three inputs: 1) a positive signal input; 2) a negative signal input; and 3) a ground input. Consequently, even the most rudimentary 1-channel EEG measurement procedure requires the use of three electrodes. Applying electrodes to the scalp takes time and skill, requires skin preparation, e.g., removal of hair, and is especially difficult in a thick hair environment.
One of the special applications for EEG which has received much attention to during the 1990""s is use of a processed EEG signal for objective quantification of the amount of brain activity for the purpose of determining the level of consciousness of a patient. In its simplest form, this usage of EEG allows for the automatic detection of the alertness of an individual, i.e. if he or she is awake or asleep. This has become a significant issue, both scientifically and commercially, in the context of measuring the depth of unconsciousness induced by anesthesia during surgery. Modern anesthesia practices use a sophisticated balancing technique with a combination of drugs for maintaining adequate hypnosis, analgesia, muscle relaxation, and/or suppression of the autonomic nervous system and blockage of the neuromuscular junction. The need for a reliable system for the monitoring of the adequacy of the anesthesia is based on both safety and economical concerns. An anesthesia dose which is too light can, in the worst case, can cause the patient to wake up in the middle of the operation and create a highly traumatic experience both for the patient and for the personnel administering the anesthesia. At the opposite extreme, the administration of too much anesthesia generates increased costs due to the excessive use of anesthesia drugs and the time needed to administer the drugs. Over dosage of anesthesia drugs also affects the quality and length of the postoperative period immediately after the operation and the time required for any long term post-operative care.
A significant main advancement in making the EEG-based measurement of the depth of unconsciousness induced by anesthesia an easy-to-use, routine procedure was a finding based on Positron Emission Tomography (PET) that determined that the effects of the anesthetic drugs on the brain are global in nature. This means that for many applications it is enough to measure the forebrain or frontal cortex EEG from the forehead of the subject. The forehead is both an easy to access and is a hairless location on the subject. Electrodes placed with an appropriate spacing between electrodes on the forehead can pick up an adequate signal originating from the anterior cortex in the brain. This discovery, together with development of a special algorithm, namely, the Bispectral Index (BIS), an electrode design requiring no skin preparation, as disclosed in U.S. Pat. No. 5,305,746, incorporated herein by reference, and a convenient integrated electrode array, as disclosed in U.S. Pat. No. 6,032,064, also incorporated herein by reference, have contributed to a viable commercial product manufactured and sold by Aspect Medical of Natick, Mass. capable of obtaining a measurement of the state or activity of the brain during delivery of anesthesia using an EEG system.
The ""064 patent teaches a disposable EEG electrode array. One array has three electrodes for 1-channel measurement. A different array has four electrodes for 2-channel measurements. The 2-channel set-up is symmetrical in configuration and separately collects the signals between the mid-forehead and left and right mastoidal points, respectively. The 2-channel measurement configuration is used to determine the differences in the EEG signal in situations in which the right and left frontal hemispheres might be expected to produce different EEG signals. This can be caused, for example, by ischemia or burst suppression, i.e., EEG signals in discontinuous bursts, in either of the sides of the head, as well as artifacts in the EEG signals due to movement of the eyes of the subject or poor contact in one of the electrodes.
However, if it is desired to switch from 1-channel to 2-channel EEG measurements, with these prior art sensors it is necessary to remove the three electrode, 1-channel sensor and replace it with a four electrode, 2-channel sensor, and vice versa. This requires significant time and effort on the part of the technician taking the measurements as the first sensor must be removed before the second sensor can be positioned on the individual, and because the positioning of the second sensor must be precise in order to obtain an accurate measurement of the neurological activity of the subject.
It would, therefore, be desirable to develop a neurological activity sensor system which is capable of operation in both a 1-channel and 2-channel manner to obtain EEG measurements of the neurological activity of the subject. The sensor system should have as simple a construction as possible to minimize the amount of time and effort necessary to properly position the electrodes of the sensor on the subject prior to obtaining the measurements.
While the foregoing has discussed the use of EEG signals, it may also be desirable to obtain electromyographic (EMG) signals arising from the forehead of the subject. Should an anesthetized patient approach a state of consciousness, the frontalismuscle in the forehead of the subject may contract from a pain sensation or for other reasons. When sensed by appropriately placed electrodes, this muscle activity can provide an early indication that the subject is emerging from anesthesia.
It is, therefore, an object of the present invention to provide a low cost sensor system of simple construction having an electrode array with three basic EEG electrodes capable of performing measurements of neurological activity in different portions of the brain, such as the overall frontal cortex of the brain or the left or right hemispheres of the forebrain.
A further object of the invention is to provide a sensor system capable of obtaining EMG signals from the head of a subject.
It is another object of the invention to provide a sensor system and method of operating same which can be configured to selectively operate in a conventional 1-channel mode or in a manner to approximate a 2-channel measurement.
It is still a further object of the invention to provide a sensor system wherein the electrode array is manufactured to be disposable.
The invention employs an electrode array of three electrodes. The sensor system uses a switching arrangement connected to the electrode array to route signals from each of three electrodes forming the array in a manner that allows measurement of the biopotential difference between any pair of the three electrodes of the system while using the remaining electrode in each case as a ground electrode. To this end, a signal from each of the three electrodes can be selected by the switching arrangement for use as a positive input signal, a negative input signal or a ground signal to a signal processing unit, such as a differential amplifier to obtain a biopotential difference used to measure the neurological or muscular activity of the subject.
The switching arrangement can route the signals from the electrodes to form a 1-channel measurement mode to monitor the neurological activity of either the left or right hemisphere of the forebrain or overall frontal cortex of the brain. The switching arrangement can also route signals from selected pairs of electrodes to the differential amplifier in a pre-determined, alternating fashion to provide an essentially 2-channel measurement of neurological activity. EMG signal data is obtained in an analogous manner.
The sensor system and method of the present invention have significant advantages compared to a fixed 1-channel set-up. First of all, the system allows for the optimization of the signal quality regarding the signal-to-noise ratio in the signals of the electrode array. The system can automatically choose to start a measurement using the electrode on the frontal hemisphere that is receiving the strongest signal and/or the least amount of noise by sampling the signals and noise levels generated by each frontal hemisphere and received by each electrode prior to starting any measurement. Secondly, by switching 1-channel measurements using selected pairs of electrode signals back and forth in a predetermined sequence, this system can also work as a surrogate for a true 2-channel measurement system. The system can also be configured to monitor the status of the electrodes, and detect the origin of any interference or signal artifacts and for the diagnosis of any physiological changes that generate lateral asymmetry in the frontal cortex neural activity, such as changes in blood flow in one of the carotid arteries.